High Signal-to-Noise Pixelated CZT Directional Spectrometer for Generalized Holdup Measurements

Holdup is the residual special nuclear material, such as enriched uranium and plutonium, that accumulates in pipes, ducts, gloveboxes and other equipment at fuel processing facilities. Quantification of this material is important for the material balance of the plant, which is a key aspect of nuclear safeguards. However, holdup is difficult to measure by hand and commercially available tools are subject to large error, primarily from the geometry of the equipment and unknown distribution of the material inside. This project seeks to design a portable instrument that can localize and help quantify the materials by imaging their gamma emissions with high fidelity. The CZT-based imaging spectrometer will use a specially designed coded-aperture mask to resolve low-energy gamma spectra as a function of direction with angular resolution of a few degrees. When left to dwell for long measurements, the image will reach high signal-to-noise ratio, improving materials quantification for complex holdup distributions. The first phase of the project will design a new imager for holdup measurements. The coded- aperture mask will be optimized using ray-tracing simulation and validated by laboratory measurements using check sources and nuclear materials resembling holdup. The measurements will be performed using an existing commercial imager with a modified mask. Directional spectroscopy software will be developed and used to experimentally test the accuracy of reconstructed spectra for quantification. Given the outcome from experiment and simulation, a prototype imager will be designed around the optimized mask and a preliminary digital model of the device will be made for future mechanical engineering development. The eventual goal of this project is to develop a portable imaging spectrometer that can deliver quantitative images to users, i.e., estimates of material mass on a pixel-wise basis. Such a tool could save workers time and effort, as well as reduce radiation dose, since they would be able to set up and leave the imager to measure a piece of equipment rather than scan it by hand. The device may also find materials that were not otherwise detectable or noticed with manual instruments. The broader effect of a successful adoption of this technology is to reduce the uncertainty in special nuclear materials balance at a plant, contributing to safeguards and civilian safety. Beyond fuel processing, this instrument may be used in other measurement scenarios requiring high-fidelity images of isotopes that emit low-energy gammas for quantification, including waste management and nuclear security.